Photobioreactor systems and methods for treating CO2-enriched gas and producing biomass

ABSTRACT

Certain embodiments and aspects of the present invention relate to a photobioreactor including covered photobioreactor units through which a liquid medium stream and a gas stream flow. The liquid medium comprises at least one species of phototrophic organism therein. Certain methods of using the photobioreactor system as part of fuel generation system and/or a gas-treatment process or system at least partially remove certain undesirable pollutants from a gas stream. In certain embodiments, a portion of the liquid medium is diverted from a photobioreactor unit and reintroduced upstream of the diversion position. In certain embodiments, the disclosed photobioreactor system, methods of using such systems, and/or gas treatment apparatus and methods provided herein can be used as part of an integrated combustion method and system, wherein photosynthetic organisms used within the photobioreactor are harvested from the photobioreactor, processed, and used as a fuel source for a combustion system such as an electric power plant.

FIELD OF THE INVENTION

The invention relates generally to photobioreactors and processes tooperate and use photobioreactors for the treatment of gases, such asflue gases, and for the production of biomass.

DISCUSSION OF THE RELATED ART

The power generation industry is coming under increasing pressure toproduce electricity from renewable energy sources. Many biofuels meetrenewable energy source standards, however, sources of conventionalbiofuels, such as biomass, biodiesel, and bioethanol, are not uniformlygeographically distributed across the nation, and in general, thesesources are not located close to power generation facilities.

At the same time, reductions in carbon dioxide emissions and other gasemissions from various sources are becoming increasingly necessaryand/or desirable. Typically, capturing carbon dioxide from the flue gasof anthropogenic sources such as electric power plants is expensive.

Photosynthesis is the carbon recycling mechanism of the biosphere. Inthis process organisms performing photosynthesis, such as plants,synthesize carbohydrates and other cellular materials by CO₂ fixation.One of the most efficient converters of CO₂ and solar energy to biomassare microalgae, often referred to herein simply as “algae,” which arethe fastest growing photoautotrophic organisms on earth and one ofnature's simplest microorganisms. In fact, over 90% of CO₂ fed to algaecan be absorbed, mostly through the production of cell mass. (Sheehan,John; Dunahay, Terri; Benemann, John R.; Roessler, Paul, “A Look Back atthe U.S. Department of Energy's Aquatic Species Program: Biodiesel fromAlgae,” 1998, NERL/TP-580-24190; hereinafter “Sheehan et al. 1998”). Inaddition, algae are capable of growing in saline waters that areunsuitable for agriculture.

Using algal biotechnology, CO₂ bio-regeneration can be advantageous dueto the production of useful, high-value products from waste CO₂.Production of algal biomass as a method of reducing CO₂ levels incombustion gas is an attractive concept because dry algae has a heatingvalue roughly equivalent to coal. Algal biomass can also be turned intoa high quality liquid fuel which is similar to crude oil or diesel fuel(“biodiesel”) through thermochemical conversion by known technologies.Algal biomass also can be used for gasification to produce highlyflammable organic fuel gases suitable for use in gas-burning powerplants. (e.g., see Reed, T. B. and Gaur, S. “A Survey of BiomassGasification” NREL, 2001; hereinafter “Reed and Gaur 2001”).

Algal cultures also can be used for biological NO_(x) removal fromcombustion gases. (Nagase Hiroyasu, Ken-Ichi Yoshihara, Kaoru Eguchi,Yoshiko Yokota, Rie Matsui, Kazumasa Hirata and Kazuhisa Miyamoto,“Characteristics of Biological NO_(x) Removal from Flue Gas in aDunaliella tertiolecta Culture System,” Journal of Fermentation andBioengineering, 83, 1997; hereinafter “Hiroyasu et al. 1997”). Somealgae species can remove NO_(x) at a wide range of NO_(x) concentrationsand combustion gas flow rates. Nitrous oxide (NO), a major NO_(x)component, is dissolved in the aqueous phase, after which it is oxidizedto NO₂ and assimilated by the algal cell. For example, NO_(x) removalusing the algae Dunaliella can occur under both light and darkconditions, with an efficiency of NO_(x) removal of over 96% (underlight conditions).

Over an 18-year period, the U.S. Department of Energy (DOE) funded anextensive series of studies to develop renewable transportation fuelsfrom algae (Sheehan et al. 1998). In Japan, government organizations(MITI), in conjunction with private companies, have invested over $250million into algal biotechnology. Each program took a differentapproach, but because of various problems addressed by certainembodiments of the present invention, none has been commerciallysuccessful to date.

A major obstacle for feasible algal bio-regeneration and pollutionabatement has been an efficient, yet cost-effective, growth system.DOE's research focused on growing algae in massive open ponds as big as4 km². The ponds require low capital input; however, algae grown in openand uncontrolled environments result in low algal productivity. The openpond technology made growing and harvesting the algae prohibitivelyexpensive, because massive amounts of dilute algal waters required verylarge agitators, pumps and centrifuges. Furthermore, with low algalproductivity and large flatland requirements, this approach could, inthe best-case scenario, be applicable to only 1% of U.S. power plants.(Sheehan et al. 1998). On the other hand, the MITI approach, withstricter land constraints, focused on very expensive closed algalphotobioreactors utilizing fiber optics for light transmission. In thesecontrolled environments, much higher algal productivity was achieved,but the algal growth rates were not high enough to offset the capitalcosts of the systems utilized. Other examples of closed photobioreactorsknown in the art include U.S. Pat. Nos. 2,732,663; 4,473,970; 4,233,958;4,868,123; and 6,827,036.

Burlew (Burlew, John S. “Algal Culture: From Laboratory to Pilot Plant.”Carnegie Institution of Washington Publication 600. Washington, D.C.,1961 (hereinafter “Burlew 1961”)) provides an overview of severaldesigns for algae bioreactors. The bioreactors discussed in Burlew 1961include the use of glass tubes, open tanks, open trenches, and coveredtrenches. In these systems, carbon dioxide is fed into a liquid via gassparging. More recently, Pulz and Scheibenbogen (Pulz O. andScheibenbogen K. “Photobioreactors: Design and Performance with Respectto Light Energy Input,” Advances in BiochemicalEngineering/Biotechnology, 59:pp 124-151 (1998); hereinafter “Pulz1998”) reviewed algae photobioreactors, and Richmond (Richmond A. ed.“Handbook of Microalgal Culture—Biotechnology and Applied Phycology,Blackwell Publishing, Oxford, UK (2004); hereinafter “Richmond 2004”)reviewed the general state of the art of microalgae culturing, includingreactor design. Both references (Richmond 2004 and Pulz 1998) note thatopen systems, such as natural lakes, circular ponds, and racewayreactors are the predominate commercial technology. Open air systemsused for cultivation of algae are also shown in, for example, U.S. Pat.Nos. 3,650,068; 3,468,057; and 4,217,728.

SUMMARY

Certain embodiments and aspects of the present invention relate to:photobioreactor apparatus; gas-treatment systems and methods employingphotobioreactors; modular components and systems for photobioreactors;methods and systems for controlling and operating photobioreactors andphotobioreactor systems; and integrated combustion/gas-treatment/carbonfuel recycling methods and systems.

According to one aspect of the invention, a photobioreactor is providedin which a gas containing elevated concentrations of carbon dioxide iscontacted with a liquid medium containing a phototrophic biologicalspecies such as algae. The gas and liquid are contained within anelongated photobioreactor unit having a light-transparent cover, and thebiological species uses the carbon dioxide and the light to grow,thereby producing biomass.

The elongated photobioreactor unit may be formed with one or moreindividual photobioreactor sections which, in the case of more than onesection, are interconnected with each other, each section having its owninlet, outlet and associated cover or portion of an integral coverspanning multiple sections. One or more of the photobioreactor unitsoperating in parallel may form a photobioreactor system. However, someembodiments of photobioreactor systems may include a singlephotobioreactor unit (formed of multiple photobioreactor sections) oreven a single photobioreactor section.

According to another aspect of the invention, a modular section-basedconstruction is used in constructing photobioreactor units such that asystem may be easily scaled depending on the needs of a particularapplication. A section-based construction also may permit interchangingof sections having different functions if operating conditions change orif adjustments to the system are desired. In certain embodiments, eachunit comprises one photobioreactor section, and a plurality of suchsections are arranged in parallel to form a photobioreactor system. Inother embodiments, one or more photobioreactor units of aphotobioreactor system may be formed from a plurality of photobioreactorsections interconnected together in series.

According to another aspect of the invention, phototrophic organisms,such as algae, and the liquid medium in which they are contained, arediverted from a photobioreactor unit and returned to the same ordifferent photobioreactor unit (or other portion of the system) upstreamof the diversion location.

According to another aspect of the invention one or more photobioreactorsections include at least one self-supporting cover which is able towithstand internal/external pressure differences and weather elements.

According to another aspect of the invention, multiple zones within thephotobioreactor system are longitudinally (i.e. in the direction ofoverall media flow) delineated and controlled to provide differentoperating conditions among the zones.

According to another aspect of the invention, components of aphotobioreactor system, such as photobioreactor units and/orphotobioreactor sections, are supported by floats on a pond or otherbody of water.

According to another aspect of the invention, liquid from one or more ofvarious stages of a photobioreactor system is used to quench incomingflue gas.

According to another aspect of the invention, wastewater from a powerplant is used to heat photobioreactors.

According to another aspect of the invention, photobioreactors includecooling zones which take advantage of evaporative cooling.

According to another aspect of the invention, a gas stream experiences alow pressure drop while passing through a photobioreactor.

According to another aspect of the invention, photobioreactors are usedover large areas which include elevation variations.

According to another aspect of the invention, a photobioreactor systemcombines the advantages of a plug flow system and a back-mixing system.

According to one embodiment of the invention, a photobioreactor systemcomprises a plurality of interconnectable photobioreactor sectionswhich, when connected together, form at least onelongitudinally-oriented photobioreactor unit of the photobioreactorsystem, the photobioreactor sections each comprising a liquid flowchannel and a light-transparent cover that forms a gas headspace betweenthe cover and the liquid flow channel. The cover is constructed andarranged to cover at least a substantial portion of the liquid flowchannel and is configured to be capable of providing the gas headspaceeven when a gas pressure within the photobioreactor unit is less thanthe atmospheric pressure surrounding the photobioreactor section.

According to another embodiment of the invention, a photobioreactorsystem capable of supporting the growth of phototrophic organisms usinga gas containing elevated levels of carbon dioxide comprises at leastone photobioreactor section constructed and arranged to carry a flow ofliquid medium comprising phototrophic organisms therein. Thephotobioreactor section comprises a cover constructed and arranged tocover at least a substantial portion of the liquid medium within thephotobioreactor section and further constructed and arranged to providea gas headspace under the cover and above the liquid medium, the coverbeing capable of providing the gas headspace even when a gas pressurewithin the photobioreactor is less than the atmospheric pressuresurrounding the photobioreactor section. The photobioreactor system alsocomprises a liquid inlet to provide liquid medium to the photobioreactorsection, a liquid outlet from which to remove liquid medium comprisingphototrophic organisms therein from the photobioreactor section, a gasinlet to provide gas containing an elevated concentration of carbondioxide into the gas headspace,

a gas outlet from which to remove gas containing carbon dioxide at aconcentration less than at the gas inlet, and a blower fluidicallyconnected to the gas outlet able to create a flow of gas through the gasheadspace from the gas inlet to the gas outlet.

According to a further embodiment of the invention, a photobioreactorsystem comprises at least one longitudinally extending photobioreactorunit comprising at least one photobioreactor section, thephotobioreactor unit being constructed and arranged to carry a flow ofliquid medium comprising phototrophic organisms therein. Thephotobioreactor unit comprises at least one cover constructed andarranged to cover at least a substantial portion of the liquid mediumwithin the photobioreactor unit and constructed and arranged to providea gas headspace under the cover and above the liquid medium, the coverbeing capable of providing the gas headspace even when a gas pressurewithin the gas headspace of the photobioreactor is less than theatmospheric pressure surrounding the photobioreactor unit. Thephotobioreactor unit further comprises a first liquid inlet constructedand arranged to provide a liquid medium to the photobioreactor unit, afirst liquid outlet from which the liquid medium is removable from thephotobioreactor unit, a second liquid outlet positioned between thefirst liquid inlet and the first liquid outlet, from which the liquidmedium is removable from the photobioreactor unit, and a channelfluidically interconnecting the second liquid outlet to thephotobioreactor unit at a position which is upstream of the secondliquid outlet to enable return and recycle of the liquid medium withinthe photobioreactor unit.

According to another embodiment of the invention, a method of growingphototrophic organisms in a photobioreactor system comprises providingliquid medium comprising phototrophic organisms therein to a firstliquid inlet of a longitudinally extending photobioreactor unit,comprising at least one photobioreactor section, such that the liquidmedium flows toward a first liquid outlet, the photobioreactor unithaving at least one rigidly supported cover. The method furthercomprises flowing a gas containing an elevated concentration of carbondioxide over the liquid medium, removing a portion of the liquid mediumfrom the photobioreactor unit at a removal position located between thefirst liquid inlet and the first liquid outlet, and returning at leastsome of the removed portion of liquid medium to the photobioreactor unitat a position upstream of the removal position.

According to a further embodiment of the invention, a photobioreactorsystem capable of supporting the growth of phototrophic organisms usinga gas containing an elevated concentration of carbon dioxide comprisesat least one photobioreactor section constructed and arranged to carry aflow of liquid medium comprising phototrophic organisms therein. Thephotobioreactor section comprises a cover constructed and arranged tocover at least a substantial portion of the liquid medium within thephotobioreactor section and constructed and arranged to provide a gasheadspace under the cover and above the liquid medium, the gas headspacebeing maintainable at a pressure that differs from atmospheric pressure.The photobioreactor section further comprises a liquid inlet configuredto provide liquid medium to the photobioreactor section, a liquid outletfrom which liquid medium is removable from the photobioreactor section,and a gas outlet from which gas containing carbon dioxide at aconcentration less than at the gas inlet is removable from thephotobioreactor section. The photobioreactor section comprises a firstportion of the photobioreactor section in which the cover provides thegas headspace over a first portion of the liquid medium, and furthercomprises a second, different portion of the photobioreactor section inwhich a second portion of the liquid medium is exposed to gas outside ofthe gas headspace to facilitate evaporative cooling of the liquidmedium.

According to another embodiment of the invention, a photobioreactorsystem capable of supporting the growth of phototrophic organisms usinga gas containing an elevated concentration of carbon dioxide comprisesat least one photobioreactor section constructed and arranged to carry aflow of liquid medium comprising phototrophic organisms therein and aflow of gas containing an elevated concentration of carbon dioxide. Thephotobioreactor system comprises a liquid inlet constructed and arrangedto provide at least liquid medium to the photobioreactor section, aliquid outlet from which liquid medium is removable from thephotobioreactor section, a gas inlet constructed and arranged to providegas containing an elevated concentration of carbon dioxide to thephotobioreactor section, a gas outlet from which gas containing carbondioxide at a concentration less than at the gas inlet is removable fromthe photobioreactor section, and a cover constructed and arranged tocover at least a substantial portion of the flow of liquid medium withinthe photobioreactor section and constructed and arranged to provide agas headspace under the cover and above the liquid medium. Thephotobioreactor section includes a portion of the photobioreactorsection where the liquid medium is exposed to gas outside of the gasheadspace to facilitate evaporative cooling of the liquid medium. Thephotobioreactor system also comprises a controller configured to controlthe amount of evaporative cooling of the liquid medium in the portion ofthe photobioreactor section where the liquid medium is exposed to gasoutside of the gas headspace.

According to yet another embodiment of the invention, a method ofremoving carbon dioxide from an effluent gas stream containing elevatedconcentrations of carbon dioxide comprises directing the effluent gasstream through a quench zone, quenching the effluent gas stream in thequench zone using a quench liquid, and directing the quenched effluentgas stream to a photobioreactor system, the effluent gas stream beingcontacted with liquid medium comprising phototrophic organisms suspendedtherein such that the phototrophic organisms use carbon dioxide from theeffluent gas stream for photosynthesis. The method further comprisesremoving at least a portion of the liquid medium comprising phototrophicorganisms suspended therein from the photobioreactor system, directingthe liquid medium to a dewatering system, dewatering the suspension ofphototrophic organisms to produce dewatered biomass and the quenchliquid, and directing quench liquid to the quench zone.

According to another embodiment of the invention, a method of removingcarbon dioxide from an effluent gas stream containing an elevatedconcentration of carbon dioxide comprises directing the effluent gasstream through a quench zone, removing liquid medium comprisingphototrophic organisms therein from a photobioreactor system, quenchingthe effluent gas stream in the quench zone using the liquid mediumremoved from the photobioreactor system, and directing the quenched gasto a photobioreactor system, the quenched gas being contacted withliquid medium comprising phototrophic organisms therein such that thephototrophic organisms use carbon dioxide from the gas forphotosynthesis.

BRIEF DESCRIPTION OF THE DRAWINGS

Other advantages, novel features, and uses of the invention will becomemore apparent from the following detailed description of non-limitingembodiments of the invention when considered in conjunction with theaccompanying drawings, which are schematic and which are not intended tobe drawn to scale. In the figures, each identical, or substantiallysimilar component that is illustrated in various figures is typicallyrepresented by a single numeral or notation. For purposes of clarity,not every component is labeled in every figure, nor is every componentof each embodiment of the invention shown where illustration is notnecessary to allow those of ordinary skill in the art to understand theinvention.

In the drawings:

FIG. 1 a is a perspective view of a photobioreactor unit according toone embodiment of the invention;

FIG. 1 b is a cross-sectional view of one photobioreactor section of aphotobioreactor unit according to one embodiment of the invention;

FIG. 2 is a perspective view of a photobioreactor system according toone embodiment of the invention;

FIG. 3 shows a block diagram of an overall gas treatment/biomassproduction system comprising a photobioreactor system according to oneembodiment of the invention;

FIG. 4 is a cross-sectional view of a nutrient misting section of aphotobioreactor unit, according to one embodiment of the invention;

FIG. 5 is a perspective view of an evaporative cooling zone of aphotobioreactor unit according to one embodiment of the invention;

FIG. 6 a is a perspective view of a first configuration of aphotobioreactor unit zone for diverting liquid to a reflow channel;

FIG. 6 b is a perspective view of a second configuration of thephotobioreactor unit zone shown in FIG. 6 a;

FIG. 7 is a perspective view of two photobioreactor unit zonesconfigured to divert liquid to a reflow channel, according to oneembodiment of the invention;

FIG. 8 a is a perspective view of one component of a bulkheaddistribution unit according to one embodiment of the invention;

FIG. 8 b is a cross-sectional view of the bulkhead distribution unitcomponent shown in FIG. 8 a;

FIG. 9 is a perspective view of a bulkhead distribution channeloperatively connected with ten photobioreactor units according to oneembodiment of the invention;

FIG. 10 is a block diagram of an overall gas treatment/biomassproduction system comprising a photobioreactor system according to analternative embodiment of the invention;

FIG. 11 is a perspective view of a photobioreactor system according toan alternative embodiment of the invention;

FIG. 12 shows a cross-sectional view of a photobioreactor unit adaptedto float on a water body;

FIG. 13 is a block diagram of an overall gas treatment/biomassproduction system comprising a photobioreactor system which uses liquidassociated with the system to quench flue gas;

FIG. 14 is a block diagram of an overall gas treatment/biomassproduction system comprising a photobioreactor system which used liquidassociated with the system to quench flue gas;

FIG. 15 is a block diagram of an overall gas treatment/biomassproduction system comprising a photobioreactor system which used liquidassociated with the system to quench flue gas.

FIG. 16 is a cross-sectional view of a quench zone according to oneembodiment of the invention;

FIG. 17 is a perspective view of the quench zone shown in FIG. 16;

FIG. 18 is a perspective view of a heat exchange zone of aphotobioreactor unit according to one embodiment of the invention;

FIG. 19 shows algae concentration versus time for one example of the useof a photobioreactor described herein; and

FIG. 20 shows carbon dioxide flux rates for embodiments employingdifferent liquid spray rates.

DETAILED DESCRIPTION

Certain embodiments and aspects of the present invention relate tophotobioreactor systems designed to contain a liquid medium comprisingat least one species of phototrophic organism therein, and to methods ofusing the photobioreactor systems as part of a gas-treatment process andsystem able to at least partially remove certain undesirable pollutantsfrom a gas stream.

Certain embodiments of the invention include one or more longitudinallyoriented, elongated covered photobioreactor units arranged in parallelthat extend across a land area or a body of water, such as a pond, toform at least a part of a photobioreactor system. In certainembodiments, each photobioreactor unit has a liquid channel (formed by atrench in some embodiments) and a gas headspace (enclosed by alight-transparent cover in some embodiments). CO₂-rich gas enters thephotobioreactor unit and flows in the headspace above a liquid mediumcomprising at least one phototrophic organism such as algae. The algaeuses the CO₂ from the gas and the light that passes through the cover togrow and produce biomass. Algae may be harvested from the liquid mediumdischarge and dewatered. The dewatered algae may go through additionalprocesses and may be used as fuel and/or used to produce a fuel product(e.g. biodiesel). The liquid produced during the dewatering phase may berecycled back into the same photobioreactor unit and/or a differentphotobioreactor unit of the photobioreactor system and/or anothercomponent of the photobioreactor system in some embodiments. In somecases, the photobioreactor units may be on the order of a few hundredfeet or less, while in other cases, the photobioreactor units may extendhalf a mile or more.

A modular, sectional construction may be used to form at least someportion of at least some of the photobioreactor units in certainembodiments. For example, in certain embodiments, a photobioreactor unitmay be made up of a plurality of individual photobioreactor sectionsinterconnected in series. In certain such embodiments, the individualsections may comprise both a liquid flow channel and at least one cover.In other embodiments, a photobioreactor unit may comprise a single,uninterrupted liquid flow channel contained in a base (e.g. base 110 ofFIG. 1 a), and the photobioreactor sections may be defined by the zonescovered by one or a subset of a plurality of cover sections (e.g. seecover sections 106 of FIG. 1 a) over the base and channel. In thismanner, the length of one or more photobioreactor units may be producedby selecting and interconnecting the appropriate number ofphotobioreactor sections, and thus custom manufacturing for specificapplications may not be required. By employing a modular construction,in some cases, the length may be adjusted after installation if desired.Additionally, various types of photobioreactor sections may be usedwithin a photobioreactor unit to create a plurality of operation zoneswith selected functionality, such as nutrient misting zones, coolingzones, liquid diversion zones, etc., and the number and positions of thevarious types of photobioreactor sections may be designed based onpredicted operating conditions. Exchanging different types ofphotobioreactor sections after installation also may be possible whenusing a modular sectional construction. In some embodiments, a largenumber of photobioreactor units may be positioned near to one another(e.g. parallel to one another), and system scaling may be achieved byadding or subtracting photobioreactor units.

In certain embodiments, the disclosed photobioreactor systems, methodsof using such systems, and/or gas treatment systems and methods providedherein can be used as part of an integrated method and system fortreating waste gasses produced by industrial processes, whereinphototrophic organisms used within the photobioreactor at leastpartially remove certain pollutant compounds contained within effluentgases, e.g. CO₂ and/or NO_(x), and are subsequently harvested from thephotobioreactor system, processed, and used as a fuel source for acombustion device (e.g. an electric power plant generator, industrialfurnace, and/or incinerator). Such uses of certain embodiments of theinvention can provide an efficient means for recycling carbon containedwithin a combustion fuel (i.e. by converting CO₂ in a combustion gas tobiomass fuel and/or biomass-derived fuel in a photobioreactor system),thereby reducing both CO₂ emissions and fossil fuel requirements.

In some embodiments, the liquid within a photobioreactor unit is sprayedinto the gas headspace or otherwise exposed to CO₂-rich gas using one ormore mass transfer enhancement devices to increase the surface-to-volumeratio of the liquid. By providing surface area contact between the gasand the liquid medium via movement of the gas through a headspace ratherthan by exclusively sparging gas into a depth of liquid medium, certainembodiments of the photobioreactor system exhibit a low pressure dropwhen moving gas through the photobioreactor units. In some embodiments,the gas pressure drop along an entire photobioreactor unit may be below0.5 psi.

In some embodiments, the flow of gas and liquid through thephotobioreactor units may experience limited or essentially no backflow,and in this way exhibit the characteristics of a plug flow system. Withlimited backflow, longitudinal zones may be defined in which differentoperating conditions such as, for example, algae density, liquidtemperature, gas composition, gas temperature, media composition, mediaagitation/turbulence, gas/liquid mass/heat transfer, light exposure,media depth, etc. are generally known and controllable by changingoperating parameters. For example, a single photobioreactor unit mayinclude different zones within which one or more of the followingoperating parameters vary and/or are known and/or are controllable:nutrient concentrations; temperature; pH; liquid depth; surface-to-airratio of the liquid; agitation levels; and others. In certainembodiments, these zones may be made up by or comprise one or morespecially configured photobioreactor sections of the photobioreactorunit.

In some embodiments, advantages of a back-mixed bioreactor may beachieved while maintaining many of the characteristics of a plug flowbioreactor. One or more reflow zones may be used to return algae-richliquid from, for example, a longitudinal mid-area of the photobioreactorunit to the front end of the photobioreactor unit or to some otherposition upstream of the liquid removal position. By doing so, theaddition of new innocula to the liquid medium at the front end of thephotobioreactor unit may be reduced or eliminated and/or other desirableoperating parameters may be maintained and/or established.

Compared to raceway reactors, which can experience considerable thermalloss when ambient temperatures are below the reactor operatingtemperature, some embodiments of the invention limit thermal loss bycovering a majority (or in some cases substantially all) of the liquidsurfaces within the photobioreactor system. Compared to typical enclosedphotobioreactors (e.g. certain tubular photobioreactors) which do notinclude a gas head space in contact with the liquid media over at leasta substantial portion of its flow length, some of which use variousmethods of thermal management to remove heat from the reactors, certainembodiments disclosed herein are able to shed heat efficiently usingcontrolled evaporative cooling.

According to certain embodiments, unlike systems that use gas pressureto support a cover, a self-supporting cover(s), e.g. rigid individualinterconnected cover section(s) or a continuous or sectioned coverformed of a flexible, non self-supporting material that comprises ribsor other support elements, may be used to maintain a gas headspaceregardless of the pressure of the gas flowing through a photobioreactorunit. The cover may be configured such that when gas is pulled through aphotobioreactor unit by an induced-draft fan, thereby creating anegative pressure within the photobioreactor unit relative toatmospheric pressure, the cover maintains the gas headspace (i.e. doesnot collapse). In some embodiments, the cover is constructed andarranged to withstand external forces such as wind and snow.

Certain aspects of the invention are directed to photobioreactor designsand to methods and systems utilizing photobioreactors. A“photobioreactor,” “photobioreactor unit” or “photobioreactor section”as used herein, refers to an apparatus containing, or configured tocontain, a liquid medium comprising at least one species of phototrophicorganism and having either a source of light capable of drivingphotosynthesis associated therewith, or having at least one surface atleast a portion of which is partially transparent to light of awavelength capable of driving photosynthesis (i.e. light of a wavelengthbetween about 400-700 nm).

The term “photosynthetic organism”, “phototrophic organism”, or“biomass,” as used herein, includes all organisms capable ofphotosynthetic growth, such as plant cells and micro-organisms(including algae, euglena and lemna) in unicellular or multi-cellularform that are capable of growth in a liquid phase (except that the term“biomass,” when appearing in the titles of documents referred to hereinor in such references that are incorporated by reference, may be used tomore generically to refer to a wider variety of plant and/oranimal-derived organic matter). These terms may also include organismsmodified artificially or by gene manipulation. While certainphotobioreactors disclosed in the context of the present invention areparticularly suited for the cultivation of algae, or photosyntheticbacteria, and while in the discussion below, the features andcapabilities of certain embodiments that the inventions are discussed inthe context of the utilization of algae as the photosynthetic organisms,it should be understood that, in other embodiments, other photosyntheticorganisms may be utilized in place of or in addition to algae. For anembodiment utilizing one or more species of algae, algae of varioustypes, (for example Chlorella, Chlamdomonas, Chaetoceros, Spirolina,Dunaliella, Porphyridum, etc) may be cultivated, alone or in variouscombinations, in the photobioreactor.

The phrases “at least partially transparent to light” and “configured totransmit light,” when used in the context of certain surfaces orcomponents of a photobioreactor, refers to such surface or componentbeing able to allow enough light energy to pass through, for at leastsome levels of incident light energy exposure, to drive photosynthesiswithin a phototrophic organism.

One embodiment of a photobioreactor unit 100 is illustrated in FIGS. 1 aand b. Liquid medium 101 flows along a trench (or, equivalently,channel) 102 within photobioreactor unit 100, and gas, such as flue gasfrom a power plant, flows through a gas headspace 104 formed betweenliquid medium 101 and a cover(s) 106 at least partially transparent tolight. Cover(s) 106 may be constructed such that gas headspace 104remains essentially constant when no gas pressure or a negative gaspressure is applied to the interior of photobioreactor unit 100.

As CO₂-rich gas flows over liquid medium 101, CO₂ dissolves into theliquid medium, and algae within the liquid medium use the CO₂ andsunlight (or other light source) to photosynthesize, grow and reproduce,thereby producing biomass. The liquid medium flows, in certainembodiments at a controlled rate, through photobioreactor unit 100, andthe algae, in certain embodiments, is harvested at an outlet ofphotobioreactor unit 100 by removing the algae-rich liquid from thephotobioreactor unit.

In some embodiments, photobioreactor unit 100 may be approximately 10meters wide and the overall photobioreactor unit 100 may be a suitablelength to process a desired amount of CO₂. In general, thephotobioreactor unit length exceed the width, and the ratio of length towidth may be greater than 100:1, and may exceed 1000:1. The gascontaining elevated concentrations of CO₂ (i.e., CO₂ concentrationswhich are higher than ambient air) may range from 1%-100%, but typicallyin the range of 4-20%. The operating pressure of the reactor maygenerally range from about 11-20 psia, preferably from 13-16 psia. Flowrates of the gas may generally range from about 0.05-50 cm/sec, or othersuitable flow rate. Liquid flow rates may generally range from about1-100 cm/sec. Biomass concentrations generally may range from 0.01-10g/l.

Several structural features of one embodiment of photobioreactor unit100 will now be described, but it is important to note that theparticular structural implementation of this embodiment are not intendedto be limiting.

Base 110 of photobioreactor unit 100 is formed of a compacted gravelbase, and cover(s) 106 is supported by structural ribs 112. Structuralribs 112 are attached to supports 114 embedded in trench sidewalls 116formed of the same material as the base (e.g., compacted gravel). Abottom liner 120 is laid over or formed within the base 110 to provide aliquid impermeable surface. Liner 120 may be, for example a plasticsheet, e.g. a polyethylene sheet, or any other suitable liner.

Cover(s) 106 may be constructed from a wide variety of transparent ortranslucent materials that are suitable for use in constructing abioreactor. Some examples include, but are not limited to, a variety oftransparent or translucent polymeric materials, such as polyethylenes,polypropylenes, polyethylene terephthalates, polyacrylates,polyvinylchlorides, polystyrenes, polycarbonates, etc. Alternatively,cover(s) 106 may be formed from glass or resin-supported fiberglass. Incertain embodiments, cover(s) 106, in certain embodiments in combinationwith support elements such as support elements 112/114, is sufficientlyrigid to be self-supporting and to withstand typical expected forcesexperienced during operation without collapse or substantialdeformation. Portions of cover(s) 106 may be non-transparent in certainembodiments, and such portions can be made out of similar materials asdescribed above for the at least partially transparent portions ofcover(s) 106, except that, when they are desired to be non-transparent,such materials should be opaque or coated with a light-blockingmaterial.

Cover(s) 106 may include a material which is UV stabilized and may, incertain embodiments be between about 4-6 mils in thickness, depending onthe material. The material, in certain embodiments in combination withsupport elements such as support elements 112/114, may be designed tosupport external loads such as snow, wind and/or negatives pressuresapplied by an induced-draft fan. Additionally, in some embodiments,cover(s) 106 may be able to withstand internal pressure, such as when aforced-draft fan is used to push gas through photobioreactor unit 100.

Each section 130 may include a separate cover 106 with each cover 106being connected to adjacent covers when the sections 130 areinterconnected. In some embodiments, each section has a support elements112/114 and a single piece of polyethylene or other suitable material isused to span multiple sections 130.

Each photobioreactor unit 100 may be formed with multiplephotobioreactor sections 130 defined, in the illustrated embodiment, byseparate cover sections 106. In this manner, constructing the designedlength of the photobioreactor unit 100 may be achieved simply byselecting and interconnecting the appropriate number of photobioreactorsections 130. In some embodiments, the length of photobioreactor unit100 may be changed and the rate of gas and/or liquid flow may be changedto accommodate long-term changes in treatment needs. Additionally,retrofitting photobioreactor unit 100 such as by increasing ordecreasing the length may be possible.

While the photobioreactor unit embodiment shown in FIGS. 1 a and 1 bincludes a trench 102 to create a liquid flow channel, in someembodiments, no trench may be present and the channel for a liquidstream may be formed at or above grade. In certain embodiments, the basecomprising the liquid flow channel may not be longitudinally continuousas illustrated, but may comprise a plurality of interconnected sections.For example, in certain embodiments, sections 130 may be defined by bothseparate a cover section and a separate base section in association witheach other. The elevation of the photobioreactor unit may besubstantially constant along the entire length of the channel orsubstantial portions thereof, and gravity flow of the liquid stream maybe induced by adding liquid to a first end of the photobioreactor unitand allowing overflow (e.g. over a wall, weir, etc.) at the oppositeend. In some embodiments, the photobioreactor unit may have a general,continuous downward pitch to promote liquid flow. In still otherembodiments, abrupt elevation drops may be provided at the junctions ofphotobioreactor sections to create liquid flow and/or a cascading effectand/or to facilitate installation and operation over land areas withmore substantial elevation changes.

Cover(s) 106 is shown as a semicircle or other curved surface in many ofthe embodiments disclosed herein, however, any suitable shape may beused, including a rectangular, triangular or trapezoidal shapes.

Referring now to FIG. 2, one embodiment of a large-scale photobioreactorsystem 200 is shown in perspective view. In this embodiment, the gasflows in the direction opposite to the liquid stream flow, however, insome embodiments, the gas may flow in the same direction as the liquidstream. Ten parallel photobioreactor units 100 are shown in theembodiment of FIG. 2, but fewer (including a single photobioreactorunit) or more photobioreactor units may be used. While photobioreactorunits 100 as illustrated comprise straight, linear segments, inalternative embodiments, one or more of the photobioreactor units may bearcuate, serpentine, or otherwise non-linear, if desired. A liquidinlet/gas outlet bulkhead 204 runs perpendicular to the photobioreactorunits at a first end of photobioreactor system 200. At an opposite endof photobioreactor system 200, a liquid outlet/gas inlet bulkhead 206also runs perpendicular to the photobioreactor units 100. An optionalrainwater drainage and vehicle access channel 208 runs parallel to theouter side of the overall photobioreactor system; however, the drainageand vehicle access channel 208 may be positioned between parallelphotobioreactor units, or may not be present at all. In someembodiments, smaller rainwater drainage channels which do notaccommodate vehicles may be provided.

The lengths of photobioreactor units 100 are selected to be sufficient,for a given desired liquid medium circulation rate, to providesufficient gas-liquid contact time to provide a desired level of masstransfer between the gas and the liquid medium. Optimal contact timedepends upon a variety of factors, especially the algal growth rate andcarbon and nitrogen uptake rate as well as feed gas composition and flowrate and liquid medium flow rate. Scalability of the photobioreactorsystem 200 as a whole may be achieved, for example, by simply by addingadditional photobioreactor units to the system, such as by addingphotobioreactor units in a parallel relationship to existingphotobioreactor units.

As described above, each photobioreactor unit 100 may include variouszones having different functionality. One or more photobioreactorsections may be configured as a misting zone 216 to controllably addnutrients/media to the system and facilitate gas-liquid mass transfer.The nutrients and/or the medium in which the nutrients are carried maybe provided in certain embodiments at least in part by recyclingalgae-depleted medium from a dewatering system. More than one nutrientmisting section 216 may be provided. By employing a modularsection-based construction, channel and/or cover sections which includemisters may be added or removed after construction if so desired. Inother embodiments, nutrients may be added by methods other than mistingsuch as by direct pumping into the liquid stream. Unrecycled nutrientsand/or medium (i.e. fresh make-up) also, or exclusively, may be used tosupply the liquid stream in some embodiments.

Of course in some embodiments, nutrients may be added using devicesother than misters. For example, nutrients may flow from a pipe into theliquid medium stream, or nutrients may be showered from the top of thephotobioreactor unit using a pipe with periodic openings.

Each photobioreactor unit 100 or certain of the photobioreactor unitsmay in certain embodiments include a cooling zone 220 comprising, incertain embodiments, cooling sections 222. As described below withreference to FIG. 5, cooling zone 220 may include portions in which theliquid stream is exposed to the atmosphere to provide for evaporativecooling.

Harvesting algae, adjusting algal concentration, and introducingadditional liquid medium can be facilitated via liquid medium inletbulkhead 204 and liquid medium outlet bulkhead 206. Control of theconcentration of algae can be important from the standpoint ofmaintaining a desirable level of algal growth and proliferation. Algaemay be harvested periodically or continuously from an end(s) of thephotobioreactor units, or, in some embodiments, from one or morelocations located between the ends of the photobioreactor units.

Various devices or mechanisms may in certain embodiments be includedwithin photobioreactor units 100 to increase the interfacial surfacearea between the gas and the liquid medium to facilitate mass transfer.Sprayers which spray the liquid medium into the gas headspace may beused in some embodiments. In some embodiments, liquid medium may bedirected onto or over sheets of plastic or other suitable material suchthat the liquid medium travels down and/or over the surfaces of thesheets and falls back into the liquid stream. Alternatively oradditionally, sheets of material which include pockets may periodicallybe dipped into the liquid stream and pulled upwardly into the gasheadspace to increase the available liquid surface area. In certainembodiments, floating objects and/or devices configured to be partiallysubmerged in the liquid medium (e.g. a paddle wheel) may be used tofacilitate enhancement of gas-liquid interfacial area and mass transfer.In certain such embodiments, the objects may be transparent such thatthey also may act to allow penetration of light to greater depths withinthe media. In some embodiments, elements may be employed to producesurface ripples or even waves that travel laterally or longitudinallywithin the liquid medium to increase mass transfer between the gas andthe liquid.

At least one or each photobioreactor unit 100 may in certain embodimentsinclude one or more diversion zones or sections 230 which divertportions of the liquid streams to at least one reflow unit such as areflow channel 232. For example, at least one channel section or zone ofa photobioreactor unit may allow liquid to flow perpendicularly to thephotobioreactor unit to reach reflow channel 232 (see FIGS. 6 a-7). Theliquid in the reflow channel may then flow toward to the liquid mediuminlet bulkhead 204 and may be added to the liquid inflow by a pump (e.g.an Archimedes screw pump). By recirculating some of the liquid mediumcomprising phototrophic organisms therein, the addition of new innoculato the liquid medium at the front end of the photobioreactor unit may bereduced or eliminated in certain embodiments. In some embodiments, therecirculation rate may generally be in the range of 0.1-0.95, andpreferably in the range of 0.5-0.7.

As would be apparent to those skilled in the art, particularconfigurations of the various photobioreactor units and components ofthe photobioreactor system will depend upon the particular use to whichthe photobioreactor is employed, the composition and quantity of the gasto be treated and other particular parameters specific to individualapplications. Given the guidance provided herein and the knowledge andinformation available to those skilled in the arts of chemicalengineering, biochemical engineering, and bioreactor design, one canreadily select certain operating parameters and design configurationsappropriate for a particular application, utilizing no more than a levelof routine engineering and experimentation entailing no undue burden.

As discussed above in the description of FIG. 2, in certain embodiments,photobioreactor system 200 can comprise a plurality of identical orsimilar photobioreactor units 100 interconnected in parallel.Furthermore, in certain embodiments, at least one or eachphotobioreactor unit may comprise one photobioreactor section or aplurality of photobioreactor sections in series. Such scalability canprovide flexibility to increase the capacity of the photobioreactorsystem and/or increase the degree of removal of particular components ofthe gas stream as a particular application or needs demand. In one suchembodiment, a photobioreactor system is designed to separate algaespecies that are efficient in utilizing NO_(x) from species efficient inutilizing CO₂. For example, a nitrogen-efficient algae is placed in afirst photobioreactor unit or a first zone of a photobioreactor unit andcarbon-efficient algae is placed in a second photobioreactor unit or ina second zone of the same photobioreactor unit in series with the firstzone. The flue gas enters the first photobioreactor unit/zone and isscrubbed of nitrogen (from NO_(x)), then flows through the secondphotobioreactor unit/zone and is scrubbed of carbon (from CO₂).

The term “fluidically interconnected”, when used in the context ofconduits, channels, chambers, or other structures provided herein thatare able to contain and/or transport gas and/or liquid, refers to suchconduits, channels, containers, or other structures being of unitaryconstruction or connected together, either directly or indirectly, so asto provide a continuous coherent flow path from one conduit or channel,etc. to the other(s) to which they are fluidically interconnected. Inthis context, two conduits or channels, etc. can be “fluidicallyinterconnected” if there is, or can be established, liquid and/or gasflow through and between the conduits and/or channels (i.e. twoconduits/channels are “fluidically interconnected” even if there existsa valve between the two conduits/channels that can be closed, whendesired, to impede fluid flow there between).

A channel or trench may comprise, in certain embodiments, fluidimpermeable wall(s) for partially or completely surrounding a fluidpassing through the channel along its direction of flow. In otherembodiments, wall(s) of a channel may only partially surround a fluidpassing through the channel along its direction of flow and/or thewall(s) may have some degree of permeability with respect to a fluidflowing in the channel, so long as the wall(s) sufficiently surround thefluid and are fluid impermeable to a sufficient extent so as to be ableto establish and maintain a bulk flow direction of fluid generally alonga trajectory parallel to a longitudinal axis or curve defining thegeometric center of the channel along its length.

The liquid medium contained within the photobioreactor system duringoperation typically comprises water or a saline solution (e.g. sea wateror brackish water) containing sufficient nutrients to facilitateviability and growth of algae and/or other phototrophic organismscontained within the liquid medium. As discussed below, it is oftenadvantageous to utilize a liquid medium comprising brackish water, seawater, or other non-portable water obtained from a locality in which thephotobioreactor system will be operated and from which the algaecontained therein was derived or is adapted to. Particular liquid mediumcompositions, nutrients, etc. required or suitable for use inmaintaining a growing algae or other phototrophic organism culture arewell known in the art. Potentially, a wide variety of liquid media canbe utilized in various forms for various embodiments of the presentinvention, as would be understood by those of ordinary skill in the art.Potentially appropriate liquid medium components and nutrients are, forexample, discussed in detail in: Rogers, L. J. and Gallon J. R.“Biochemistry of the Algae and Cyanobacteria,” Clarendon Press Oxford,1988; Burlew 1961; and Round, F. E. The Biology of the Algae. StMartin's Press, New York, 1965; each incorporated herein by reference).

FIG. 3 schematically shows one embodiment of a gas treatment/biomasproduction/photobioreactor system 300 that uses solar energy andphotobioreactor system 200 comprising photobioreactor units 100 toproduce biomass using a flue gas containing elevated concentrations ofcarbon dioxide (i.e., gas having a concentration of carbon dioxidegreater than ambient air). Flue gas is sent from a CO₂ source 302 to agas conditioner 306, such as a conventional quench zone know to one ofskill in the art, to reduce the gas temperature and possibly removeharmful species such as acid gases. In certain embodiments, a forceddraft fan 308 may be used to facilitate this transfer of flue gas and/orpush gas through photobioreactor units 100, but in some embodiments noforced draft fan is used. The gas is then sent through thephotobioreactor units 100 so that the carbon dioxide (and potentiallyother gases) can interact with a liquid stream in the photobioreactorunits to generate biomass. Photobioreactor system 200 may be constructedof one or more photobioreactor units 100 as described above. In theembodiment shown in FIG. 3, the gas is flowed countercurrently to theliquid stream, that is, the liquid stream flow from liquid inlet/gasoutlet bulkhead 204 to liquid outlet/gas inlet 206. Make-up liquidmedium (not shown) may be added during operation. In some embodiments,for example as described below with reference to FIG. 10, the flow ofgas may be co-current with the liquid stream flow.

The photobioreactor units 100 may include different zones, e.g. 218,219, 220, 221, along the lengths of the various photobioreactor units.In some embodiments, each photobioreactor unit may have similar zones,while in other embodiments, different zones and/or different zonelocations may be provided in various of the photobioreactor units. Forexample, in a first zone 218, the bioreactor may include nutrientaddition capabilities such as nutrient misting facilities. A second zone219 may provide the option of diverting a portion of the liquid flowfrom the main photobioreactor units so that it may be returned to anupstream zone. Third zone 220 may include cooling capabilities such asevaporative cooling. A fourth zone 221 may be designed and/or controlledto environmentally stress algae, for example to increase lipidsproduction. It should be noted that these particular zones are providedby way of example only, and as described further below, photobioreactorsystem 200 and/or individual photobioreactor units withinphotobioreactor system 200 may include fewer or more zones.

CO₂-depleted gas exits photobioreactor units 100 through liquidinlet/gas outlet bulkhead 204 and may be vented to the atmosphere orpassed to further treatment options. An induced-draft fan 312 may beused to pull gas through the bioreactor, or, as described above, aforced-draft fan 308 may be used upstream of the photobioreactor units100 instead of or in addition to the induced-draft fan in someembodiments. By using an induced-draft fan, the photobioreactor systemand/or other portions of the overall system may be maintained at anegative pressure, thereby reducing the risk of unintentional venting ofuntreated gases to the atmosphere. Additionally, the use of aninduced-draft fan (e.g., a blower), may simplify the integration of aphotobioreactor system with existing power plants thereby reducingdisruptions to power plant operations. A blower is considered to befluidically connected to a photobioreactor unit even if it is notdirectly connected to the photobioreactor unit, that is, other pieces ofequipment or other conduits may be connected between the photobioreactorunit and the blower.

In certain embodiments, a portion of the liquid stream may be diverted,as shown by arrow 318, from a downstream zone of the photobioreactorunits 100 and returned to an upstream zone (or in some embodiments toliquid inlet/gas outlet bulkhead 204) which may provide some of thebenefits of a “back-mixed” reactor system. In this regard, the amount ofinoculum added to the liquid in the photobioreactor units may be reducedor eliminated. Additionally, overall average residence time for theliquid medium may be increased without extending the length of thephotobioreactor units. The diverted liquid medium may be returned at aposition and in a manner such that the returned liquid medium causes orincreases turbulence in the liquid stream, which may enhance heating orcooling and/or photomodulation in certain photobioreactor unit sections.

As mentioned above, photobioreactor units 100 also may include a coolingzone(s) 220 such as an evaporative cooling zone. In some embodiments,while flowing through photobioreactor unit 100, the liquid streamtemporarily exits the enclosed portion of the photobioreactor unit andis exposed to the atmosphere. Evaporation of some of the liquid coolsthe remaining liquid, which then reenters the enclosed portion of thephotobioreactor unit 100. Each photobioreactor unit may be constructedand arranged such that the liquid stream does not significantly changedirection or speed when exiting and reentering the enclosed portion ofthe photobioreactor unit. For example, as shown in FIG. 5, one or morephotobioreactor sections of a photobioreactor unit may include wallsthat reduce the amount of cross-sectional area available for gas flow,but provide an area where the cover section(s) may be removed orindented, as shown, to allow exposure of the liquid stream to theatmosphere.

In some embodiments of evaporative cooling zones, a portion of theliquid stream may be continuously exposed to the atmosphere, that is,for a relatively long zone of the photobioreactor unit, which may bemade up of a large number of photobioreactor sections, the zone, or eachsection comprising such zone, may include an area (for example on thelateral side of the trench) that provides an evaporative cooling area.Substantially continuous mixing of the exposed portion of the liquidstream with the unexposed portion of the liquid stream may provideadequate cooling for the reactor.

The photobioreactor sections and/or units may be heated and maintainedat certain temperatures or temperature ranges suitable or optimal forproductivity. These specific, desirable temperature ranges for operationwill, of course, depend upon the characteristics of the phototrophicspecies used within the photobioreactor systems, the type ofphotobioreactor, etc. Typically, it is desirable to maintain thetemperature of the liquid medium between about 5 degrees C. and about 45degrees C., more typically between about 15 degrees C. and about 37degrees C., and most typically between about 15 degrees C. and about 25degrees C. For example, a desirable temperature operating condition fora photobioreactor utilizing Chlorella algae could have a liquid mediumtemperature controlled at about 30 degrees C. during the daytime andabout 20 degrees C. during nighttime. In one embodiment, the temperatureof the photobioreactor is maintained at about 20 degrees C.

In certain embodiments, the temperature, velocity, residence time,depths and/or nutrient concentrations can be maintained at differentlevels/values in the various zones to control for different factorsand/or provide particular functionality. For example, it is possible incertain embodiments to maintain one zone so as to maximize growth ratesand to maintain conditions in another zone to maximum lipids production.

Algae-rich liquid exiting from photobioreactor system 200 may be sent toa dewatering system 322. Various conventional methods and/or systems ofdewatering may be used to dewater the algae, including dissolved airfloatation and/or tangential flow filtration, or any other suitabledewatering approach.

The dewatered algae may be sent for further processing 324, for example,drying. Dried algal biomass can be used directly as a solid fuel for usein a combustion device or facility and/or could be converted into a fuelgrade oil (e.g., biodiesel) and/or other fuel (e.g., ethanol, methane,hydrogen). The algae also may be used as food supplements for humans andanimals. In certain embodiments, at least a portion of the biomass,either dried or before drying, can be utilized for the production ofproducts comprising organic molecules, such as fuel-grade oil (e.g.biodiesel) and/or organic polymers. Methods of producing fuel grade oilsand gases from algal biomass are well known in the art (e.g., see, Dote,Yutaka, “Recovery of liquid fuel from hydrocarbon rich micro algae bythermo chemical liquefaction,” Fuel. 73:Number 12. (1994); Ben-ZionGinzburg, “Liquid Fuel (Oil) From Halophilic Algae: A renewable Sourceof Non-Polluting Energy, Renewable Energy,” Vol. 3, No 2/3. pp. 249-252,(1993); Benemann, John R. and Oswald, William J., “Final report to theDOE: System and Economic Analysis of Micro algae Ponds for Conversion ofCO₂ to Biomass.” DOE/PC/93204-T5, March 1996; and Sheehan et al., 1998;each incorporated by reference).

Algae-depleted medium resulting from dewatering operations may bedisposed of or may be returned to photobioreactor system 200 (afteroptionally being mixed with fresh liquid medium), as shown by arrow 328,to return unused nutrients to the system. Such an approach may reducethe amount of fresh water and nutrients to be added to the system.

In some embodiments, other processes of the photobioreactor system maybe integrated with the power plant or other CO₂ source. For example, thehot flue gas from the power plant may be used to at least partially drythe biomass produced by the photobioreactor system.

Algae, or other phototrophic organisms, may, in certain embodiments, bepre-adapted and/or pre-conditioned to specific environmental andoperating conditions expected to be experienced in a full scalephotobioreactor system of the invention during use. Methods andapparatus for adaptation and pre-conditioning algae may be found incommonly-owned International Application Publication No. WO 2006/020177,which is hereby incorporated by reference in its entirety.

Although photobioreactor system 200 is described as being utilized withnatural sunlight, in alternative embodiments, an artificial light sourceproviding light at a wavelength able to drive photosynthesis may beutilized in supplement to or instead of natural sunlight. For example, aphotobioreactor utilizing both sunlight and an artificial light sourcemay be configured to utilize sunlight during the daylight hours andartificial light in the night hours, so as to increase the total amountof time during the day in which the photobioreactor system can convertCO₂ to biomass through photosynthesis.

Since different types of algae can require different light exposureconditions for optimal growth and proliferation, in certain embodiments,especially those where sensitive algal species are employed, lightmodification apparatus or devices may be utilized in the construction ofthe photobioreactors according to the invention. Some algae specieseither grow much more slowly or die when exposed to ultraviolet light.If the specific algae species being utilized in the photobioreactor issensitive to ultraviolet light, then, for example, certain portions ofcover(s) 106, or alternatively, the entire cover outer and/or innersurface, could be coated or covered with one or more light filters thatcan reduce transmission of the undesired radiation. Such a light filtercan readily be designed to permit entry into the photobioreactor systemof wavelengths of the light spectrum that the algae need for growthwhile barring or reducing entry of the harmful portions of the lightspectrum. Such optical filter technology is already commerciallyavailable for other purposes (e.g., for coatings on car and homewindows). A suitable optical filter for this purpose could comprise atransparent polymer film optical filter such as SOLUS™ (manufactured byCorporate Energy, Conshohocken, Pa.). A wide variety of other opticalfilters and light blocking/filtering mechanisms suitable for use in theabove context will be readily apparent to those of ordinary skill in theart. In certain embodiments, especially for photobioreactor systemsutilized in hot climates, as part of a temperature control mechanism, alight filter comprising an infrared filter could be utilized to reduceheat input into the photobioreactor system, thereby reducing thetemperature rise in the liquid medium.

Referring now to FIG. 4, one embodiment of a nutrient/medium mistingphotobioreactor section or zone 400 is illustrated. A liquid inlet 402may be formed of a conduit that also provides support for a mister 404.In some embodiments, liquid may flow into inlet 402 and all of theliquid may exit through mister 404. In some embodiments, liquid may flowthrough inlet 402 and some of the liquid may exit through mister 404while the remaining liquid exits through an outlet 406 on the oppositeside of section or zone 400 and continues to an adjacent photobioreactorunit. Mister 404 is shown as spraying liquid downwardly in FIG. 4, butin some embodiments the liquid may be aimed upwardly toward the insideof cover 106, such as directly upwardly. In this manner, mister 404 orother liquid injection device may help to clean the inside of cover 106and the thin film of liquid formed on the inside surface of the covercan further enhance gas-liquid mass transfer.

FIG. 5 shows a perspective view of one embodiment of a cooling zone 220for a photobioreactor unit 100. In this embodiment, cover(s) 106 formsthree walls 502, 503, 504 which reduce the cross-sectional area of thegas headspace. Each wall 502, 503, 504 penetrates into liquid stream 101such that photobioreactor unit 100 remains gas-tight. Walls 502, 503,504 may not, however, in certain embodiments reach the base ofphotobioreactor unit 100, such that, therefore, in such embodiments, theliquid stream may readily flow into evaporative cooling area 508. Insome embodiments, sprayers 510 or other devices which increase surfacearea exposure of the liquid stream to the atmosphere may be employed toenhance evaporative cooling.

While evaporative cooling area 508 is shown to be present only on oneside of the photobioreactor unit in this embodiment, a secondevaporative cooling area may additionally (or instead) be provided onthe opposite side of the photobioreactor unit, or positioned at anintermediate location positioned between the two laterally opposed sidesof photobioreactor unit 100. For embodiments in which cooling zone 220comprises one or more interconnectable photobioreactor sections, as withphotobioreactor sections that include nutrient misters for embodimentsincluding such photobioreactor sections, the interchangeability of thephotobioreactor sections may allow for the addition or subtraction ofcooling areas after installation of the photobioreactor system.

One embodiment of a liquid flow diversion photobioreactor section orzone 230 is illustrated in FIGS. 6 a and 6 b. As shown in FIG. 6 a, amovable weir 240 may be deployed such that all liquid in thephotobioreactor unit liquid stream 101 is directed through bypassconduits 242. In such a configuration, none of the liquid flowingthrough diversion photobioreactor section or zone 230 is diverted, andall of the liquid medium flowing through the section continues towardthe liquid medium outlet. With the movable weir 240 lowered, as shown inFIG. 6 b, a portion of the liquid medium is diverted into a transversechannel 244 which flows to a reflow channel such as reflow unit 232illustrated in FIG. 2. In some cases, all of the liquid stream isdiverted depending on the relative heights of bypass conduits 242,adjustable weir 240 and the liquid levels in trench 102 and transversechannel 244. In certain embodiments, the degree of diversion iscontrollable either or both of manually or through use of a computeroperated process control system.

A controller, e.g. a computer-implemented system, may be used to monitorand control the operation of the various components of thephotobioreactor sections, units and systems disclosed herein, includingvalves, sensors, weirs, blowers, fans, dampers, pumps, etc. Certainembodiments may employ computer systems and methods described incommonly-owned International Publication No. WO2006/020177, particularlywith reference to FIG. 7A of that publication. In addition to automatingoperation of aspects of the photobioreactor system, use of acomputer-implemented system may facilitate optimizing or improving theefficiency of the system by determining suitable values for variouscontrol parameters. In some embodiments, flow may be controlled toprovided a desired level of turbulence and light/dark exposure intervalsfor improved growth, and described and determined according to methodsalso described in International Publication No. WO2006/020177.

FIG. 7 shows another embodiment of a diversion photobioreactor sectionor zone 230. In this embodiment, an adjustable weir 250 may be loweredto allow liquid medium to flow into transverse channel 244. Whenadjustable weir 250 is raised, the liquid medium flows through a bypassportion 254 of diversion zone 230 to continue along the photobioreactorunit.

One embodiment of a liquid and gas bulkhead zone 600 is shown in FIGS. 8a-8 b. In certain embodiments, a series of sections 600 may be connectedend to end and travel transversely to a plurality of parallelphotobioreactor units, as shown in FIG. 9. Each bulkhead section 600 mayinclude an automated weir 601 or other liquid control element foradjustably controlling the size and elevation of a liquid passageway602. Each bulkhead section 600 also may include a flue gas damper 603 orother flue gas control element for controlling the size of a gaspassageway 604. An embossing 606 or ridge for attachment to aphotobioreactor unit may be provided on a side of bulkhead section 600.The sizes of liquid passageway 602 and gas passageway 604 may be fixedor adjustable. For example, in a system with a consistent liquid streamflow rate, the weirs for each of a plurality of photobioreactor unitsmay be permanently set such that flow from the bulkheads issubstantially equal for each photobioreactor unit. In other embodiments,each bulkhead section may include an adjustable weir so that the flow ofliquid to each photobioreactor unit can be independently controlled.Similarly, gas passageways may be designed to equally distribute gasflow amongst all of the photobioreactor units, or, gas dampers may beconfigured and/or operated so that gas flow to each photobioreactor unitmay be independently controlled. At least one cover 610 for the bulkheadsection(s) may be transparent and otherwise similar to the covers forthe photobioreactor units, or, in some embodiments, the cover may beopaque and/or made of a different material than the photobioreactor unitcovers.

Ten bulkhead sections 600 are shown interconnected in FIG. 9 to form abulkhead distribution unit 700. The open lateral inlet 701 to the gashead space of the bulkhead provides an inlet for flue gas that may befluidically interconnected with a conduit(s) supplying feed gas from aCO₂ source and/or gas conditioner 306 and/or quench zone of the system(discussed below). Recirculated liquid 702 from a reflow channel 232 isshown being pumped into bulkhead distribution unit 700. The recirculatedliquid 702 mixes with fresh liquid medium and/or liquid being recycledfrom dewatering operations, and the liquid is distributed to the variousphotobioreactor units 100 by gravity flow through liquid passageways602.

Although not shown, dampers, such as guillotine dampers, between one ormore bulkhead sections may be used to limit gas and/or liquid flow tocertain photobioreactor units. A guillotine damper and/or other flowcontrol element may also be used within a single point entry to thebulkhead region so that all flow of gas and/or liquid may easily bestopped.

While many of the embodiments described herein employ the movement ofliquid through a gas headspace to promote mass transfer between the gasand liquid, in certain embodiments, additionally or alternatively, gasmay be sparged into the liquid. For example, while the bulk of gasdistribution into the liquid medium present in a photobioreactor unit100 may be through a gas passageway such as the one shown in FIG. 1 a, anot insignificant amount of gas may be sparged into the liquid medium incertain embodiments. The sparging, in addition to creating an additionalgas-liquid interface, may create turbulence or additional turbulence incertain regions where such turbulence is desirable.

In an alternate embodiment of the invention, a photobioreactor systemmay include some or all of the elements of the photobioreactor systemshown and described in FIG. 3, with the exception of the recycle forrecirculating liquid from downstream in a photobioreactor unit toupstream in the photobioreactor unit. FIG. 10 shows one embodiment ofsuch a system, which may include many of the same elements as the systemdescribed above with reference to FIG. 3. Additionally, FIG. 10illustrates an embodiment in which gas flows co-currently with liquidflow through photobioreactor units 100. Thus, both liquid and gas flowfrom a liquid inlet/gas inlet bulkhead 340 to a liquid outlet/gas outletbulkhead 342 in this embodiment.

A perspective view of one physical embodiment of the photobioreactorsystem 700 illustrated in FIG. 10 is shown in FIG. 11.

In many current photobioreactor systems, chosen, desirable strains ofalgae can be difficult to maintain in a photobioreactor that is notscrupulously sterilized and maintained in a condition that is sealedfrom the external environment. The reason for this is that the algalstrains being used in such photobioreactors are not well adapted oroptimized for the conditions of use, and other, endemic algal strains inthe atmosphere are more suitably conditioned for the local environment,such that if they have the ability to contaminate the photobioreactorthey will tend to predominate and eventually displace the desired algaespecies. Such phenomena may be mitigated and/or eliminated by usingadaptation protocols and algal cultures described in InternationalPublication No. WO2006/020177 A1, published on Feb. 23, 2006, which ishereby incorporated herein in its entirety. Use of such protocols andalgae strains produced by such protocols may not only increaseproductivity and longevity of algal cultures in real photobioreactorsystems, thereby reducing capital and operating costs, but also mayreduce operating costs by reducing or eliminating the need to sterilizeand environmentally isolate the photobioreactor system prior to andduring operation, respectively.

Many power plants include ponds or other bodies of water to which wasteheat is discharged. In some embodiments, especially in colder climates,a photobioreactor may be positioned on top of a wastewater pond toachieve one or more possible advantages. By floating or otherwisepositioning a bioreactor on a body of water, the photobioreactor systemmay take advantage of the inherent flatness of the surface of a body ofwater over an expansive area. Further, by using an already existingpond, limited additional geographic area is required for thephotobioreactor system. If the body of water accepts heated wastewaterfrom the power plant (or other source) the photobioreactor system can beheated by the body of water to improve biomass production and/or preventfreezing in cold ambient conditions.

One embodiment of a photobioreactor unit 800 adapted for positioning ona body of water is shown in FIG. 12. Photobioreactor unit 800 issupported by two pontoon floats 802 that extend longitudinally along thelength of the photobioreactor unit. Of course, other structures may beused to float or support one or more photobioreactor units on a body ofwater.

To accommodate rain water and/or melted snow runoff, a drain system (notshown) may be incorporated into any of the above describedphotobioreactor systems. In one embodiment of a drainage system, adrainage hole is provided periodically along a collection channelpositioned between two photobioreactor units of the photobioreactorsystem. The drainage hole empties into a drainage conduit thattransversely spans each of the photobioreactor units that are positionedside-by-side. The drainage conduit leads to a drainage trench to leadwater away from the photobioreactor system. In some embodiments, thedrainage trench may be wide enough to accommodate various vehicles (e.g.vehicle access channel 208 of FIG. 2 may comprise a drainage trench).

In certain embodiments, advantageously, hot flue gas being received froma power plant may be cooled and/or scrubbed to remove undesirablecomponents with liquid that is used as part of the photobioreactorsystem according to some embodiments of the invention. For example, asillustrated in FIG. 13, in some embodiments of a gas gas treatmentsystem 900, algae-free medium that results from dewatering operationsmay be sprayed in a quench zone 902 to cool/scrub hot flue gas beforethe gas enters photobioreactor system 200 comprising photobioreactorunits 100. Liquid effluent from quench zone 902 may be disposed of, orin some embodiments, returned to photobioreactor units 100 (dashedline).

Using liquid medium to quench the flue gas heats the medium and mayreduce the pH of the medium. One or both of these effects may help killadventitious biological species, such as rotofers, cilitates, bacteria,and viruses that may impair the growth of the desired algae. If thequench effluent stream is returned to the system upstream of thedewatering step, it may improve the dewatering operation. For example,reducing the pH of the dewatering feed may improve the effectiveness ofpolycationic coagulants and alum-based flocculants. Additionally,thermally heating the algae-containing media may induce necrosis andautoflocculation, which simplifies the dewatering process and may reduceor eliminate the need for chemical additives.

In an alternative embodiment illustrated in FIG. 14, algae-rich mediumharvested from the outlet of photobioreactor units 100 may be used inquench zone 902 to cool flue gas. The liquid effluent from quench zone902 may then be sent to dewatering system 322 to enrich the algae. Aswith some other embodiments described herein, algae-free medium fromdewatering system 322 may optionally be returned to photobioreactorunits 100.

In a further embodiment of a photobioreactor system including quenching,illustrated in FIG. 15, enriched algae from dewatering system 322 may beused to cool hot flue gas in quench zone 902. Dewatered algae may beapproximately 3% solids concentration after primary dewatering, and10-20% solids after secondary dewatering. Using dewatered algae inquench zone 902 may help to stabilize the algae against decomposition,preheat the algae to aid in downstream processing, and allow somecomponents to react with the acid gases, which may promote downstreamprocesses such as fermentation.

In some embodiments, an integrated system for performing an integratedcombustion method may include a photobioreactor system whereincombustion gases are treated with the photobioreactor system to mitigatepollutants and to produce biomass, for example in the form of harvestedalgae which can be used as a fuel for the combustion device and/or forthe production of other products, such as products comprising organicmolecules (e.g. fuel grade oil (e.g. biodiesel) and/or organicpolymers). Further description of such an integrated system, which canbe used in conjunction with embodiments of photobioreactor systemsdisclosed herein, may be found in commonly-owned PCT Publication No.WO2006/020177 A1, published on Feb. 23, 2006, commonly-owned U.S. PatentApplication Publication Nos. US-2005-0064577-A1 and US-2005-0239182-A1,and PCT Application No. US2005/025249, filed on Jul. 18, 2005, each ofwhich is hereby incorporated by reference in its entirety.

One embodiment of a configuration for quench zone 902 is illustrated inFIG. 16. In this embodiment, spray elements 904 extend perpendicularlyto a liquid supply conduit 906 and are configured spray liquid into agas headspace 908. Liquid effluent is collected from the bottom of atrench 910 and either disposed of or recycled back into thephotobioreactor system. A perspective view of one embodiment of quenchzone 902 in FIG. 17 illustrates that spray elements 904 may be sprayconduits 914 including longitudinal slits.

In some embodiments of the invention, waste heat (in the form of heatedwater) may be used to heat liquid media in a photobioreactor system. Oneembodiment of tubes 920 submerged in liquid medium 101 is shown in FIG.18. Tubes 920 in FIG. 18 may continue longitudinally within the samephotobioreactor section or unit, and/or may continue laterally toadjacent photobioreactor sections or units. In some embodiments, jets922 may be used to increase the flow rate of liquid medium 101 pasttubes 920 to increase the rate of heat transfer.

PROPHETIC EXAMPLES Example 1

In this example, a laboratory test of an embodiment of a photobioreactorof the present disclosure is compared to a model of the same. Algaespecies Nannochloris sp. is grown in a 20 cm depth of Media 1, which issea water comprising 0.075 g/l NaNO₃ and 0.00565 g/l NaH₂PO₄.2H₂O. Thegrowth rates for the algae as a function of time, concentration, andlight intensity, measured as photon flux, can be derived from laboratorytests with well-stirred open tanks fed with gas containing 5 mol % CO₂and the balance O₂ and N₂ in a 1:5 molar ratio. The test results areshown in FIG. 19 for insolation rates of 2000, 1000, and 750 μE/m²·s,and the productivity is tabulated in Table 1. As shown in FIG. 8, theproductivity is not a function of concentration in this operating range.Independently, the growth rate can be predicted following the methods ofWu and Merchuk, (A Model Integrating Fluid Dynamics in Photosynthesisand Photoinhibition Processes. Chemical Engineering Science56:3527-3538, 2001). The parameter μ_(max) was averaged 0.077 hr-1 induplicate tests, and parameter kx is taken as 0.22 m2/g per Oswald (TheEngineering Aspect of Microalgae. In: Laskin, I., and Lechevalier, H.A., Editors. CRC Handbook of Microbiology. Cleveland CRC Press, pp519-552, 1977.) The model productivities matched the measuredproductivities well, as shown in Table 1.

TABLE 1 Algae Growth Rate Model Predicted Light Intensity MeasuredProductivity Productivity (μE/m2-s) (dry weight g/m2-hr) (dry weightg/m2-hr) 2000 1.4 1.4 1000 1.1 1.1 750 0.7 0.9

The bioreactor gas/liquid exchange is measured in a flowing rectangularconduit with 5 mol % CO₂ flowing above a media containing base so thatCO₂ uptake can be measured by carbonate analysis in the liquid phase.The results are shown in FIG. 20, expressed as CO₂ flux (mmol/m²-sec) vspH of the media. Recycled media from dewatering is used to enhance theCO₂ gas-liquid exchange. FIG. 20 also show the enhanced gas-liquid masstransfer rates that can be achieved by spraying the recycled media intothe headspace of the reactor for two different spray rates, normalizedto the reactor area. The test results illustrate the increase in CO₂transfer rates which can be obtained by properly re-injecting thedewatering fluid into the reactor. These higher CO₂ transfer rates canreduce the bioreactor area requirements in situations where the algalproductivity is limited by gas mass transfer. Alternatively these higherCO₂ transfer rates can be used to increase the total biomass productionrates from a bioreactor of fixed size.

A covered bioreactor is modeled using the algal growth model discussedabove and the mass transfer rates from the gas-liquid tests. Thebioreactor has a depth of 20 cm and a liquid velocity of 20 cm/sec toensure a high level of turbulence. The bioreactor is sufficiently longthat the flow is essentially plug flow; i.e. the Peclet number is high.The liquid phase comprises Media 1 maintained at pH 7.8 with an initialalgae recycle rate to maintain the algae concentration ofin the feed endat 0.1 g cell dry weight/liter. The flue gas contains 5 mol % CO₂, andflows through channels with a gas freeboard height of 2 m. Thebioreactor is covered with polyethylene plastic film, with a measuredvisible light transmission of 95%. The media recycled from thedewatering system is split with 80% returned to the bioreactor toenhance the CO₂ mass transfer rate, and 20% sent to the open areas ofthe bioreactor to generate a spray that enhances liquid cooling. Theambient dry-bulb temperature is assumed to be 30° C., with a wet-bulbtemperature of 25° C. The reactor productivity, CO₂ conversion, powerrequirements for the flue gas handling and water consumption are listedin Table 2 for three levels of solar insolence. Table 2 also shows

TABLE 2 Comparison of Bioreactor Performance at 30 C. Ambient ReactorCO₂ Power Water Light Intensity productivity conversion requirementConsumption Example (μE/m2-s) (g/m2-hr) (mol %) (kW) (kg/m2-hr) Example1 - 2000 1.4 60% 1 1.1 Present Embodiment 1000 1.1 50% 1 .5 750 0.9 40%1 .4 Example 2 - 2000 1.4 20% 30 1.1 Raceway Pond

Example 2

This example illustrates the advantage of embodiments ofphotobioreactors disclosed herein compared to a conventional racewaypond. The reactor productivity, CO₂ conversion, power requirements forthe flue gas handling, and water consumption are listed in Table 3 forthe highest level of solar insolence using the same operating conditionsof Example 1, based on published values for CO₂ conversion andevaporation rates. Flue gas is sparged into a 2-meter deep well in theraceway via a blower that compresses flue gas to 8 psig. The resultsshow that the hybrid bioreactor achieves comparable growth rates, whileattaining greater CO₂ conversion and using substantially less power. Theraceway pond power consumption is significantly higher due to its lowerCO₂ capture efficiency, requiring higher flue gas flows per unit ofalgae produced, and its higher pressure drop. Water consumption for bothreactors is comparable because both use evaporative cooling to maintainreactor temperature.

Example 3

This example illustrates the advantage of the hybrid open/closedbioreactor at a lower ambient temperature, 5° C., compared to a racewaypond. The system of Examples 1 and 2 is operated at identicalconditions, with the exception that none of the recycled media ofExample 1 is directed towards the cooling zone, and low-level heat fromthe power plant condenser cooling loop is used to maintain thebioreactor temperature. Table 3 lists the productivity and heat duty formaintaining 25° C. in the two reactors. The results show that thisreactor has significant advantages over an open raceway pond.

TABLE 3 Comparison of Bioreactor Performance at 5° C. Ambient LightReactor Intensity productivity Heat duty Example (μE/m2-s) (g/m2-hr)(kW/m2) Example 3 - 1000 1.1 0.02 Photobioreactor Example 3 - 1000 1.10.10 Raceway pond

Example 4

This example illustrates options for integrating the dewateringoperation with the bioreactor. Nannochloris sp. is grown in Media 1 atbioreactor temperatures ranging from 17-27 C, with insolation ranging upto about 2200 μE/m2-s. The biomass concentration ranges from 0.2 to 10grams/liter. The algae is dewatered using the techniques known in theart as Dissolved Air Flotation. The feed to the dewatering system ismixed with aluminum sulfate to attain a concentration of 50-300 ppm inthe media, and contacted with bubbles generated by dissolving air intothe filtrate that is recycled to the dewatering unit at a 10% rate. Thealgal biomass creates a floe that is 4-5 wt % solids. Essentiallyalgae-free filtrate is recycled to the reactor, allowing unreactednutrients to be returned to the system. Recycling this stream reducestotal water and nutrient requirements. Optionally a portion or all ofthe dewatering feed stream can be contacted with flue gas in the quenchzone prior to dewatering. For flue gases containing acid gases such asSO₂, NO_(x), and HCl, absorption of the acid gases reduces pH fromapproximately 7-9 range to a more preferred range of 6.5-7.5. In this pHrange, the quantity of aluminum sulfate required to dewater the algae isreduced.

Example 5

This example illustrates the use of Tangential Flow Filtration fordewatering the algae. The algae of Example 5 is run in a system usingtangential flow filtration instead of dissolved air floatation. Thefiltration process uses a sterile-grade membrane and operates at lowtrans-membrane pressure and low shear rates to increase the algaeconcentration by a factor of 10-200. Cellular debris and bacterialcontaminants are concentrated with the algae-rich stream. The sterilizedpermeate stream is recycled to the reactor, conserving water andnutrients while reducing risk due to recycle of deleterious species suchas bacteria and cell lysates.

Example 6

This example illustrates the use of different operating conditionsupstream and downstream of the algae recycle point(s) to affect changesin the algae growth rates and algae composition. The bioreactor ofExample 1 is operated with the algae recycle zone located ⅔ down thelength of the reactor channel. Recycled media is used to add nitratesuch that the concentration in the feed end is 0.075 g/l and theconcentration in the recycle stream is 0.03 g/l. In the zone downstreamof the algae recycle stream split, the recycled media contains nutrientssuch as phosphate, but no nitrate. The algae in the first zoneexperience growth rates of 1.4 g/m²-hr, and lipid content isapproximately 14 wt %. The algae in the second, nitrate-poor regiondemonstrate lower growth rates, but have lipids content that exceeds 14wt %.

While several embodiments of the invention have been described andillustrated herein, those of ordinary skill in the art will readilyenvision a variety of other means and structures for performing thefunctions and/or obtaining the results or advantages described herein,and each of such variations, modifications and improvements is deemed tobe within the scope of the present invention. More generally, thoseskilled in the art would readily appreciate that all parameters,dimensions, materials, and configurations described herein are meant tobe exemplary and that actual parameters, dimensions, materials, andconfigurations will depend upon specific applications for which theteachings of the present invention are used. Those skilled in the artwill recognize, or be able to ascertain using no more than routineexperimentation, many equivalents to the specific embodiments of theinvention described herein. It is, therefore, to be understood that theforegoing embodiments are presented by way of example only and that,within the scope of the appended claims and equivalents thereto, theinvention may be practiced otherwise than as specifically described. Thepresent invention is directed to each individual feature, system,material and/or method described herein. In addition, any combination oftwo or more such features, systems, materials and/or methods, providedthat such features, systems, materials and/or methods are not mutuallyinconsistent, is included within the scope of the present invention. Inthe claims (as well as in the specification above), all transitionalphrases or phrases of inclusion, such as “comprising,” “including,”“carrying,” “having,” “containing,” “composed of,” “made of,” “formedof,” “involving” and the like shall be interpreted to be open-ended,i.e. to mean “including but not limited to” and, therefore, encompassingthe items listed thereafter and equivalents thereof as well asadditional items. Only the transitional phrases or phrases of inclusion“consisting of” and “consisting essentially of” are to be interpreted asclosed or semi-closed phrases, respectively. The indefinite articles “a”and “an,” as used herein in the specification and in the claims, unlessclearly indicated to the contrary, should be understood to mean “atleast one.”

As used herein in the specification and in the claims, the phrase “atleast one,” in reference to a list of one or more elements, should beunderstood to mean at least one element selected from any one or more ofthe elements in the list of elements, but not necessarily including atleast one of each and every element specifically listed within the listof elements and not excluding any combinations of elements in the listof elements. This definition also allows that elements may optionally bepresent other than the elements specifically identified within the listof elements that the phrase “at least one” refers to, whether related orunrelated to those elements specifically identified. Thus, as anon-limiting example, “at least one of A and B” (or, equivalently, “atleast one of A or B,” or, equivalently “at least one of A and/or B”) canrefer, in one embodiment, to at least one, optionally including morethan one, A, with no B present (and optionally including elements otherthan B); in another embodiment, to at least one, optionally includingmore than one, B, with no A present (and optionally including elementsother than A); in yet another embodiment, to at least one, optionallyincluding more than one, A, and at least one, optionally including morethan one, B (and optionally including other elements); etc. In caseswhere the present specification and a document incorporated by referenceand/or referred to herein include conflicting disclosure, and/orinconsistent use of terminology, and/or the incorporated/referenceddocuments use or define terms differently than they are used or definedin the present specification, the present specification shall control.

1. A photobioreactor system comprising: a plurality of interconnectablephotobioreactor sections which, when connected together, form at leastone longitudinally-oriented photobioreactor unit of the photobioreactorsystem, the photobioreactor sections each comprising a liquid flowchannel, and a light-transparent cover that forms a gas headspacebetween the cover and the liquid flow channel, the cover beingconstructed and arranged to cover at least a substantial portion of theliquid flow channel and configured to provide the gas headspace evenwhen a gas pressure within the photobioreactor unit is less than theatmospheric pressure surrounding the photobioreactor section, at leastone photobioreactor unit of the photobioreactor system further includingan evaporative cooling area, including a reservoir and a sprayer, theevaporative cooling area being disposed outside of the cover such thatthe reservoir is open to the atmosphere outside of the cover, thereservoir being in fluid communication with the liquid flow channel, thesprayer_configured to spray a liquid upwardly from within the reservoir.2. A system as in claim 1, wherein a first subset of the plurality ofinterconnectable photobioreactor sections comprises a differentfunctionality than a second subset of the plurality of interconnectablephotobioreactor sections.
 3. A system as in claim 1, wherein thephotobioreactor system comprises a plurality of photobioreactor units.4. A system as in claim 3, wherein the plurality of photobioreactorunits are arranged in parallel.
 5. A system as in claim 4, wherein theplurality of photobioreactor units each include the same number ofsections.
 6. A system as in claim 1, wherein the liquid flow channelcontain a liquid medium comprising phototrophic organisms.
 7. A systemas in claim 6, wherein the phototrophic organisms comprise algae.
 8. Asystem as in claim 1, further comprising a gas inlet in fluidcommunication with the gas headspace, wherein the gas inlet is connectedto a source of waste gas having an elevated concentration of CO₂.
 9. Asystem as in claim 8, wherein the waste gas comprises a flue gas.
 10. Asystem as in claim 1, further comprising a dewatering system in fluidcommunication with an output of the at least one longitudinally-orientedphotobioreactor unit.
 11. A system as in claim 1, further comprising amister disposed within a photobioreactor section of the plurality ofphotobioreactor sections and configured to spray a liquid within the gasheadspace.
 12. A system as in claim 11, wherein the mister is configuredto spray the liquid downwardly towards the liquid flow channel.
 13. Asystem as in claim 1, wherein the cover is supported by ribs attached tothe base.
 14. A system as in claim 1 further comprising two pontoonfloats that extend longitudinally along a length of the photobioreactorunit and configured to support the photobioreactor unit on a body ofwater.